Glob folder

Glob reads files and folders, but Plumber doesn't play well if folders are globbed. It tries to read them as files: https://github.com/plumberjs/plumber/blob/master/lib/util/supervisor.js#L82

I saw in the TODO file your example of `glob('src')`. AFAICS this would just be a matter of reading the contents of a directory if one is globbed, here: https://github.com/plumberjs/plumber/blob/master/lib/util/supervisor.js#L82

Do you have any other thoughts before I have a go?